Enhancing the Resistance of Soybean Plants to Waterlogging Stress Using Gamma-Irradiated Priestia megaterium

The rhizosphere of Fabaceae contains plant growth-promoting rhizobacteria (PGPR) that regulate phytohormone balance, nutrient acquisition, and biofilm formation, thereby contributing to plant development and stress tolerance. Although Indole-3-acetic acid (IAA) and exopolysaccharides (EPS) are well-established PGPR-associated factors, their combined contributions to waterlogging tolerance in soybean remain unclear. In this study, Bacillus-like PGPR were isolated from the rhizosphere of soybean plants cultivated in field soil, and Priestia megaterium and P. aryabhattai were identified among Bacillus-like bacteria based on strong EPS production, a PGPR-associated trait. The P. megaterium isolate S135-A1-2 was subjected to gamma-ray irradiation to generate mutants, from which mutants #6 (increased EPS), #8 (increased IAA), and #12 (increased EPS and IAA) were selected for further analysis. Mutant #8 produced 4.48 μg/mL IAA, a 3.6-fold increase over the parental strain, and protease and siderophore activities were also evaluated as additional PGPR-associated traits. Their plant growth-promoting traits were evaluated using pot assays under waterlogging conditions. Inoculation with mutants #6 and #8 increased lateral root development and root biomass relative to the parental strain, whereas mutant #12 showed a distinct response pattern associated with dual-trait enhancement under waterlogging stress. After 7 days of waterlogging, mutant #12 showed higher chlorophyll-related values than mutants #6 and #8. These results indicate that gamma-irradiated P. megaterium mutants can differentially modify soybean root system responses to flooding stress, supporting gamma-ray irradiation as a useful approach for developing PGPR with improved functions for agricultural applications under waterlogging stresses.